In presently known brake cylinder devices having the self-adjusting feature to compensate for brake shoe or pad wear, a pawl and ratchet mechanism is arranged in the brake cylinder cooperatively with a sleeve member coaxially surrounding the piston rod and axially movable relative thereto. The amount of relative axial movement between the piston rod and the sleeve member is initially adjusted to an initial amount reflecting the correct clearance between the brake pads and braking surfaces on a vehicle wheel. With each successive application of the brakes, a certain amount of wear occurs on the pads, thereby causing a progressively corresponding increase in the amount of relative axial travel between the piston rod and the sleeve member, until the amount of such wear, and therefore the relative axial travel between the piston rod and sleeve member, is sufficient to cause the pawl and ratchet to make an incremental adjustment thereof. With such incremental adjustment, the amount of relative axial travel between the piston rod and the sleeve member is restored to the initial amount and, therefore, the correct clearance between the brake pads and the wheel surface is restored.
The type of self-adjusting arrangement for a brake cylinder device, as immediately above described, is objectionable in certain respects in that each adjustment is incremental, that is, the adjustment occurs after a number of brake applications until the wear on the pads has accumulated sufficiently to effect the adjustment. Thus, the brake applications occurring between points of adjustment do not provide the most efficient braking action. Moreover, due to the arrangement of the pawl and ratchet, the pawl, being spring biased into engagement with the toothed ratchet, exerts a radial force on the sleeve member which, in turn, transmits such radial force to the piston on which the sleeve member is slidably supported. The radial force thus acting on the piston transversely to and during axial movement thereof causes undesirable peripheral and uneven wear on the outer cylindrical surface of the piston.